In general, this invention relates to the field of interferometry and, in particular, to the high accuracy measurement of aspherical surfaces and wavefronts in an absolute manner.
Aspheric surfaces offer significant advantages to the designer of optical systems, but their use has been constrained by difficulties in manufacture and measurement. Next generation lithography tools are being designed with aspheric mirrors and lenses with apertures approaching 0.5 meters with tolerances on the surface shapes of less than a nanometer. One approach to providing the measurements necessary during the production of these optical elements is to use a Fizeau interferometer with an aspheric reference surface such as that described in U.S. Provisional Patent Application No. 60/299,512 filed on Jun. 20, 2001 in the name of Carl A. Zanoni and entitled xe2x80x9cAPPARATUS AND METHOD FOR MEASURING ASPHERICAL OPTICAL SURFACES AND WAVEFRONTSxe2x80x9d, which was converted to regular U.S. patent application Ser. No. 10/152,075 filed on May 21, 2002 and published on Jan. 1, 2003 as US-2003-0002048-A1. The reference surface of such an aspheric Fizeau must itself be calibrated, typically in a separate instrument. Dismounting the reference element from the calibration instrument and remounting it in the production tester (Fizeau) may lead to deformations and poses risks during handling. Further, this approach to calibration measures only the reference surface, not the entire instrument performance. Another disadvantage of separate calibration schemes is that the difficulties they pose dissuade instrument users from frequent recalibration (remastering). When measurement uncertainties of a small fraction of a nanometer are required, it is advantageous to remaster the system frequently.
Interferometric testing of aspheric optics using xe2x80x9cnull correctorsxe2x80x9d is widely performed, as the fabricators of the Hubble telescope taught (inadvertently), but this approach simply transfers the calibration problem from the optic to the null. Approaches that do not require a null have long been sought.
Michael Kxc3xcchel in U.S. Pat. No. 5,004,346 issued on Apr. 2, 1991 and Mark J. Tronolone, et al. in U.S. Pat. No. 5,416,586 issued on May 16, 1995 describe techniques where an aspheric surface (or wavefront) is characterized by scanning with respect to a spherical wavefront and collecting data in zones. Uncertainty in the measurement arises, inter alia, from uncertainty in the lateral coordinate at which each zone is collected. New methods and analysis introduced by Michael Kxc3xcchel in U.S. Provisional Patent Application No. 60/303,856 filed on Jul, 9, 2001 and entitled xe2x80x9cSCANNING INTERFEROMETER FOR ASPHERIC WAVEFRONTS AND WAVEFRONTSxe2x80x9d, converted to U.S. patent application Ser. No. 10/180,286 filed on Jun. 26, 2002 and published on Mar. 6. 2003 as US-2003-0043385-A1, eliminate this problem: Kxc3xcchel in the foregoing provisional patent application shows the scanning of aspheric surfaces with respect to spherical wavefronts, and vice versa. All scanning techniques, however, are relatively slow. Zanoni in the foregoing U.S. Provisional Patent Application No. 60/299,512, now U.S. patent application Ser. No. 10/152,075 (US-2003-0002048-A1) introduced the concept of using an aspheric reference surface in a Fizeau interferometer to allow rapid, robust measurement of the difference between the reference and a test optic; such a scheme requires a technique for calibrating the reference.
It is an object of this invention to provide in-situ calibration of the performance of an aspheric Fizeau interferometer.
It is a further object of this invention to integrate the systems needed for calibration into an aspheric Fizeau to facilitate rapid, frequent remastering.
Other objects of the invention will be obvious and will appear hereinafter when the detailed description is read in connection with the accompanying drawings.
Interferometric apparatus and methods by which aspheric surfaces and wavefronts may be precisely measured. The apparatus is provided with two modes of operation. In one mode, the apparatus is configured generally as a Fizeau interferometer in which an aspheric reference surface is used to permit the rapid, robust measurement of the difference between the aspheric reference surface and an aspheric test optic or wavefront. In another mode of operation, the aspheric test surface itself is completely characterized through in-situ use of an interferometric scanning technique using a spherical reference surface.
In one aspect, the invention comprises an interferometric method for measuring rotationally and non-rotationally symmetric optics. The method comprises the steps of: scanning at least one aspheric reference surface with a spherical reference surface to determine the shape of the aspheric reference surface; aligning a test optic to be measured with the aspheric reference surface placed closely adjacent the test optic by a predetermined distance; illuminating the at least one reference aspherical surface with a beam having a predetermined wavefront such that part of the predetermined wavefront is reflected from the reference aspherical surface to serve as reference wavefront and part is transmitted as a continuing aspherical wavefront that impinges on the test optic and returns therefrom as a measurement wavefront that travels back towards the reference aspherical surface; and combining the reference and measurement wavefronts to form an interferogram containing phase information indicative of the shape of the wavefront generated by the test optic.
In another aspect, the invention comprises an interferometric apparatus for measuring rotationally and non-rotationally symmetric test optics. The interferometric apparatus comprises: means for mounting a test optic to be measured so that it can be illuminated as it is precisely aligned; means for scanning an aspheric reference surface to precisely determine its shape; means for aligning the reference aspherical surface closely adjacent the test optic to be measured and spaced therefrom by a predetermined distance; and means for illuminating the reference aspherical surface with a beam having a predetermined wavefront whose shape is substantially the same as that of the reference aspherical surface so that part of the predetermined wavefront is reflected from the reference aspherical surface to serve as reference wavefront and part is transmitted as a continuing aspherical wavefront that impinges on the test optic and returns therefrom as a measurement wavefront that travels back towards the reference aspherical surface, the reference and measurement wavefronts combining to form an interferogram containing phase information indicative of the shape of the wavefront generated by the test optic.